1 Cambridge 2004 Wolfgang Baumjohann IWF/ÖAW Graz, Austria With help from: R. Nakamura, A. Runov, Y. Asano & V.A. Sergeev Magnetotail Transport and Substorms

2 Sun-Earth Connection

3 Standard Model Magnetospheric convection is driven by solar wind.  Merging of dipolar field with southward IMF at MP  Open field lines move tailward over polar cap  Reconnection to dipolar & SW field lines in tail  Closed field lines move sunward in equatorial plane

4 Inner Magnetosphere  GEOS-2 Electron Beam Experiment measures "shift" of gyration circle of 1 keV electrons by electrical drift  Southward IMF: convection towards magnetopause  Northward IMF: only weak plasma flow  Mean values for southward IMF correspond to standard model In inner magnetosphere quasi-static convection:

5  Dependence of convection velocity toward plasma sheet on polarity of IMF B Z clearly visible Lobe Convection - 1  Cluster Electron Drift Instrument (EDI) uses same principle as GEOS-2 Electron Beam Experiment  Cluster/EDI gives first direct measurements of convection in lobe (measuring electric field in extremely thin plasma over polar cap)

6 Lobe Convection - 2  EDI data also show IMF B Y effect  Shear flow in Y-Z plane

7 Magnetotail Pressure Catastrophe:  Adiabatic convection: d/dt PV  = 0, V =  B -1 ds  Flux tube volume strongly decreases toward Earth  Convection stops to avoid pressure catastrophe Tail observations with IMP show:  Fast Earthward flow for R E  Closer in, convection severely slows down

8 Near-Earth Neutral Line  Tail-like field geometry weakens pressure gradient  Reconnection leads to smaller flux tube volume  Earthward convection by bursty bulk flows  Reversal of fast flow direction observed by Geotail  Near-Earth neutral line located ~25 R E

9 Flow & Curv B Reversal  Magnetic field components in Cluster barycentre: 4 current sheet traversals  Field line curvature: curv B = (b.grad)b  Flow and field line curvature reversal  X-line moves tailward over Cluster

10 Reconnection & Hall Effect  Ion Flow Reversal during 4 neutral sheet crossings  X-line moves tailward over Cluster  500 km thin CS around X-line  bifurcated current sheet on both sides  Hall effect (  B y ) during ‘outer’ crossings shows ion decoupling

11 Electron Hall Current  Cluster 2003 tail passes can resolve fine structure of currents  J Y shows very thin current sheet (triple peaks?)  J X consistent with electron Hall current in ion diffusion region

12 Currents at PSBL and X-line  Strong flow shear (N-S electric field) and thin field aligned current layer (1500 km) during a substorm  Consequence of Hall-effects in reconnection region and closure of the Hall-electric current Ion diffusion region

13 Braking & Dipolarization  Intermittent high- speed Earthward flow bursts (~500 km/s)  Bursty fast flows accompanied by dipolarization  Fast flow braked near 10 R E by dipolar field  More dipolar flux added by flow  Pressure gradients lead to current wedge and aurora

14 Aurora & Electrojet  Aurora (in upward field-aligned current region electrons are accelerated downward)  Substorm electrojet (adds to convection electrojet) Flow braking and flow shear generate:

15 Flow Channel Width  Cluster gives direct estimate of spatial scale of bursty bulk flows  Vertical: R E, Azimuthal: 2-3 RE  Sharper gradient on duskside flank

16 Aurora and Bursty Bulk Flow  Isolated flow bursts with E>2mV/m (Geotail) always correspond to auroral activations (Polar).  Auroral activations near foot point of satellite start within 1 min of flow burst onset.

17 Structure of Flow Bursts Spatial scale of flows Small expansion, pseudo-breakup 1.4 MLT  (4-5 R E ) Auroral streamer (N-S aurora) 0.7 MLT  (3-4 R E )  Flow bursts are centered 0.4 MLT east of aurora

18 Flow Bursts and FAC  Scale size consistent with ionospheric observations  Aurora corresponds to upward FAC in bubble model

19 Substorm Recovery  45 min after onset dipolarization front meets neutral line  No reconnection in dipolar field; recovery phase begins  Neutral line retreating tailward Fast flows transport dipolar field inward:

20 Summary Scenario  Plasma energy dominant outside of 25 R E  Flow uninhibited  Magnetic energy increases near R E  Pending pressure catastrophe leads to NENL  Magnetic field dominant inside 15 R E  Dipolar field brakes BBF: current wedge & aurora are generated  Dipolarization front travels downtail and meets NENL  near-Earth reconnection stops

21 Thanks…